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Ubiquitin is a small protein consisting of 76 amino acids that is important for the regulation of protein function in the cell (Lill J R et al., Pharmacological Sciences 2014 April; 35(4): 187-207). In addition to ubiquitin, there are a growing number of structurally related ubiquitin-like molecules (UBLs) that modify substrates in parallel but distinct cellular pathways. These proteins include but are not restricted to small ubiquitin-like modifier (SUMO), interferon-stimulated gene 15 (ISG15), ubiquitin-related modifier-1 (URM1), neuronal-precursor-cell-expressed developmentally downregulated protein-8 (NEDD8), human leukocyte antigen F-associated (FAT10), autophagy-8 (ATG8) and -12 (ATG12), few ubiquitin-like protein (FUB1), membrane-anchored UBL (MUB), ubiquitin fold-modifier-1 (UFM1) and ubiquitin-like protein-5 (UBL5). Ubiquitylation and deubiquitylation of ubiquitin and UBLs are enzymatically mediated processes by which ubiquitin or UBLs are covalently bound or cleaved from a target protein by ubiquitylating enzymes and deubiquitylating enzymes (DUBs). Including the Sentrin specific proteases (SENPs), there are approximately 95 DUBs in human cells, divided into sub-families based on sequence homology, the largest of these being the USP family that is characterised by a common Cys and His box containing Cys and His residues critical for DUB activity. The ubiquitylation and deubiquitylation processes have been implicated in the regulation of many cellular functions including cell cycle progression, apoptosis, modification of cell surface receptors, regulation of DNA transcription and DNA repair. Thus, the ubiquitin system has been implicated in the pathogenesis of numerous disease states including inflammation, viral infection, metabolic dysfunction, CNS disorders, and oncogenesis.
USP7 is a DUB with strong oncology links and is an established anti-cancer target. The rationale behind targeting USP7 is principally due to its well validated role in regulating multiple oncogenes, tumour suppressors, viral proteins and epigenetic regulators including Phosphatase and tensin homolog (PTEN), Forkhead box protein O4 (FOXO4), the p53:HDM2 axis and DNA (cytosine-5)-methyltransferase 1 (DNMT1) (Nicholson B et al., Cell Biochem. Biophys. 2011; 60:61-68). Inhibition of USP7 causes degradation of Human double minute 2 homolog (HDM2), stabilisation of p53 and activation of apoptosis in tumour cells meaning it is a potential target for cancers where there is deregulated HDM2 expression (˜7% of all cancers) and/or wild-type p53 (˜50% of all cancers). In addition, USP7 inhibition has also been shown to reduce the immune-suppressive capacity of regulatory T-cells (Van Loosdregt J et al., Immunity 2013 Aug. 22; 39:259-271, Laurence A et al., Immunity 2013 Aug. 22; 39:201-203). Thus, USP7 inhibitors may have synergistic anti-cancer effects by boosting surveillance and killing of cancer cells by the host immune system.
The ubiquitin-proteasome system has gained interest as a target for the treatment of cancer following the approval of the proteasome inhibitor bortezomib for the treatment of multiple myeloma. Extended treatment with bortezomib is limited by its associated toxicity and drug resistance. However, therapeutic strategies that target specific aspects of the ubiquitin-proteasome pathway upstream of the proteasome, such as DUBs, are predicted to be better tolerated (Ndubaku C et al., J. Med. Chem. 2015 Feb. 26; 59(4):1581-95). Although there is strong interest in this field, DUB inhibitors have yet to enter the market (Kemp M, Progress in Medicinal Chemistry 2016; 55:140-192). Thus, there is a need for compounds and pharmaceutical compositions to inhibit DUBs such as USP7 for the treatment of indications where DUB activity is observed, including, although not limited to, cancer.